The use of membranes for gas separation is becoming increasingly more common. In these systems, a mixture of gases under relatively high pressure are passed across the surface of a membrane adapted to act as a selective barrier, permitting some components of the gas mixture to pass through more readily than others. The separation of gases in these processes is generally due to molecular interaction between the gaseous components of the feed stream. Because different components interact differently with the membrane, their transmission rates through the membrane are different, and substantial separation of components can be effected. While a certain selective effect can result from free molecular diffusion through membrane pores, especially in the case of small gas molecules such as hydrogen and helium, membrane separation is often considered to proceed principally by the sorption of a gaseous component on the feed side of the membrane, diffusion of that component through the membrane, and desorption of the component from the permeate side of the membrane. Membranes used for gas separation processes wherein the separation mechanism is controlled principally by solubility and diffusivity, as opposed to molecular diffusion, are classified as nonporous membranes. While these nonporous membranes do in fact have small pores, they are typically produced in a carefully regulated manner to provide a dense layer which effectively controls the gas transfer in the system. The structure of this dense control layer is often crucial to membrane performance, and it can be adversely affected by such factors as moisture, chemical degradaton, or physical deformation.
Gas transfer through nonporous membranes is dependent upon the membrane surface area, the pressure differential across the membrane, the diffusion rate of the gaseous components, and the effective thickness of the membrane. Generally, the membrane layer through which the gases must diffuse should be as thin as possible in order to obtain the maximum amount of gaseous diffusion. However, the membrane thinness is limited by a need to have a membrane free from defects, such as pinholes, and the need to have a membrane which has the physical integrity to withstand pressures as high as about 4,000 pounds per square inch-gauge (psig) through the membrane. For example, asymmetric cellulose ester membranes can be produced which do have a very thin but dense (nonporous) layer and a supporting sublayer of larger pore size. The thin dense layer basically controls the mass transfer in the system, and the thicker sublayer provides a degree of structural integrity. Many types of membranes, including cellulose esters and polymeric membranes, such as silicate rubber, polyethylene and polycarbonate, may be employed in gas separation. However, the particular membrane used can depend upon the separation sought to be effected.
Commerical gas separation processes are generally continuous areas in which a feed gas stream is brought into contact at the feed side of a membrane. The pressure on the feed side of the system is maintained at a pressure sufficiently higher than the pressure on the permeate side of the membrane to provide a driving force for the diffusion of the most permeable components of the gaseous mixture through the membrane. The partial pressure of the more permeable gaseous components is also maintained at a higher level on the feed side of the membrane than on the permeate side by constantly removing both the permeate stream and the residue of the feed stream from contact with the membrane. While the permeate stream can represent the desired product, in most gas permeation processes the desired product is the residue stream, and the permeate stream consists of contaminants which are removed from the feed stream.
For example, CO.sub.2 and H.sub.2 S can be removed from a hydrocarbon mixture, such as natural gas, using a thin dried supported cellulose ester membrane, and a differential pressure across the membrane of about 100 psi. The partial pressures of CO.sub.2 and H.sub.2 S in the permeate stream are preferably kept at about 80 percent or less of the partial pressure of those same components in the feed stream by separately and continuously removing the depleted feed gas (residue) stream and the permeate stream from contact with the membrane. The residue stream can, of course, be fed to another gas separation membrane stage, and the permeate gas stream can likewise be fed to another separation stage to produce a product having a still higher concentration of the more permeable products. In fact, the use of multiple separation steps in series and/or in parallel offers considerable diversity in separation alternatives using membrane technology so long as sufficient pressures can be maintained in the system. Feed stream pressures can vary from 10 to 4,000 psig, but are generally within the range of about 500 psig to about 3,000 psig. The differential pressure across the membrane can be as low as about 10 pounds per square inch (psi) or as high as about 2,100 psi depending on many factors, such as the particular membrane used, the flow rate of the inlet stream, and the availability of a compressor to compress the permeate stream, if such compression is desired. A differential pressure of at least 100 psi is preferred since lower differential pressure may require more modules, more time, and compression of intermediate product streams of modules arranged in series. Differential pressures of 1,200 psi or less are also generally preferred since the useful lfe of membranes is generally greater. Differential pressures greater than about 2,100 psi may rupture the membrane. Although additional membrane support may be provided by porous metal or plastics, these materials can significantly affect the size of the system, and they can also create additional problems of compatibility, especially under conditions where they expand or contract to a different degree than the membrane itself.
Spiral wound membrane arrangements are becoming more commonly used in commercial gas separation processes. An advantage of using a spiral wound technique is that this affords a large membrane contact area while permitting a rather small overall containment vessel. A standard way of supplying spiral wound membranes for commercial use is in the form of membrane units which comprise a section of permeate conduit around which the membrane is wound. These membrane units may then be used singly or joined together in series by interconnecting their permeate conduit sections. The usual way to use spiral wound membrane units is to contain them, either singly or multiply in modules. The modules can then in turn be used singly or can be conveniently interconnected in series or parallel arrangements to provide the desired treatment.
The useful life of gas separation membranes, including in particular spiral wound membranes, has not been entirely predictable. Various factors are believed to affect the performance of membranes over time. These include the normal operating pressure differentials, the character of the gas being treated, and the quality of the membrane itself. Membranes can also degrade naturally over time. A continuing challenge for those seeking to use gas separation membrane systems has been to improve the reliability of membrane systems, especially by prolonging the useful life of the membranes used therein.
It has now been observed that although membranes are designed to withstand considerable pressure differential where the feed side pressure exceeds the permeate side pressure, their asymmetric structure, especially when spirally wound, makes them more succeptible to damage when system pressure is reversed. For example, spiral wound elements which ae designed to withstand 1,200 psi differential pressure during normal operation, have been adversely affected by relatively small reverse pressure differentials. Indeed, permeate pressures which exceed feed pressures by more than 5 psi have severely damaged, or even ruptured, membrane elements. These reverse pressure conditions may occur instantaneously, or otherwise, particularly in high pressure systems, due to plant upset.